Light valve regulation



May 11. 1937. L. T. GOLDSMITH LIGHT VALVE REGULATION Filed Aug. 15, 193; 2 Sheets-Sheet n m m m E x n P n N s N ON W 0 M n u a T n m m M u n n u n m m m w 1" m 1 ow m m n u m u u r m m A. n u W n n 2 A m u m m m FI i May 11, 1937. L. T. GOLDSMITH LIGHT VALVE REGULATION Filed Aug. 15, 1933 2 Sheets-Sheet 2 A TTORN Patented May 11, 1937 UNITED STATES LIGHT VALVE REGULATION Lloyd T. Goldsmith, Hollywood, Calif., assignor to United Research Corporation, Long Island City, -N. Y., a corporation of Delaware Application August 15, 1933, Serial No, 85,241

' V 14 Claims. (01'. re-490.3 I

The invention relates to the regulation of the light valve employed for modulating a beam, of light ,inaccordance with electrical impulses, such as electrical impulses varying in accordance with sound waves to be recorded. 1

It has heretofore been proposed to bias the light valve in accordance with the sound waves being recorded, to reduce the reproduced ground noise of the film, the light valve slowly opening and closing in accordance with the amplitude of the impulses being recorded. The idealoperation of this arrangement presupposes two things which do not occur in practice, namely that the intensity of the light source remains constantand also that the normalvalve opening remains constant It has been found that the sourceof light may decrease in intensity due to the gradual formation of a metallic deposit on the inside of the lamp bulb, and also the normal valve opening may change due torecording of a very loud sound Which causes the light valve ribbons to clash and become deformed. It is a simple matter for the operator to determine, during recording operations, if the lamp for the light valve is extinguished, but it is practically impossible with the arrangements heretofore available, for the operator during recording operations to detect minute variations in the intensity of the light source or to determine if the light valve is clashing or that the clashing has resulted in a variation of the normal light valve opening.

An object of the invention is to overcome these defects and to make it possible for the operator during recording operationstoimmediately detect any variation in the intensity of' the light source and any variation in "the normal valve opening, including the valve clash. The invention is also useful for initially preparing the apparatus for sound recording, in providing a ready method and means for determining whether the intensity of the light source and the valve opening have normal values, whereby if they have not, the intensity of the lamp current may be properly adjusted or a new lamp substituted to establish proper conditions of lightintensity and the bias on the valve may be changed to establish the proper normal valve opening.

For further details of the invention reference may be made to the drawings, wherein:

Fig. 1 is a schematic circuit diagram of a sound recording arrangement according to the invention; l I

Fig. 2 is a side elevation of alight valve recorder embodying the circuit arrangement of Fig. 1.'

Fig. 3 is a sectional view on line 3--3 of Fig. 2.

' Fig. 4 is aside elevation of the cabinet shown in-thelower part of, Fig. 3, with the door of the cabinet open to illustrate a convenient way of housing and mounting the circuit of thisinvention.

Fig. 5 is an enlarged View of the dial of Fig. 4,

this dial indicating prior to or during sound recording operations, the valve opening in thousands of an inch, the corresponding noise reduction in decibels also being indicated on this dial.

Referring to Fig. 1, the light source I of constant intensity furnishes a beam of light which is focused by the lenses 2 and 3 onto film 4, this beam of light being modulated by the light valve ribbons 5, 6 as is well understood in the art. The 1 ribbons 5 and 6 arelocated in a magnetic field, not illustrated, as is well understood, these ribbons being supplied from the line -1 with the impulses to be recorded. In other words, the line i is connected to anamplifier and a microphone as is well known. Also some of the audio impulses being recorded are taken from line I, through amplifier 8, transformer 9, rectifier ID, to supply by means of an obvious circuit, impulses for changing the bias of the light valve for noise 5 reduction purposes, as is well understood. The normal bias on the valve 5, 6 is adjusted by varying the amount of resistance ii in series with the biasing battery l2, which supplies a steady bias current to the light Valve. As can be seen, the output of the rectifier I0 is also connected across the light valvebut with a reversed polarity so that the rectified audio impulses derived from rectifier, Ill tend to reduce the steady bias current. Rectifier i3 is so poled with reference to battery 12 that current from the battery can fiow through the light valve but current with opposite polarity from rectifier 10 cannot. Consequently the rectifiedaudio impulses from rectifier ill canreduce .the'steady bias current to zero, but cannot force a current through the valve in the opposite direction.

Furthermore, as is well understood in the art, for monitoring purposes, some of the light beam being recorded, is reflected by mirrors l4 and I5 onto a photo-electric cell l6, which is connected through a suitable amplifier IT to an outgoing line [8 connected to a loud speaker not illustrated. The photo-electric cell l6 has connected directly in circuit with the electrodes thereof, a high resistance IQ of about one half megohm, battery 20 and high resistance 2| of about ten megohms. The high resistance l9 furnishes energizing impulses to the amplifier H, the upper terminal of resistance [9 being connected to the grid of the first tube 22 over an obvious circuit, the cathode of this tube being grounded at 23, and the lower terminal of resistance |9 leading through battery 20 to the ground 24.

It has heretofore been common practice to employ a photo-electric cell, such as l6, having in circuit therewith high resistance I9 and battery 20 to energize a monitoring loud speaker through an amplifier such as IT. As the amplifier I! does not constitute applicants invention, it will not be described in further detail.

According to the present invention, the position and/or motion of the light valve ribbons 5, 6 are translated into movement proportional to the average valve opening. In the case illustrated, the movement just referred to, is that of the needle of the meter illustrated in Fig. 5 and indicated at 25 in Fig. 1. This meter 25 is calibrated in several ways hereafter described and is energized by a suitable amplifier such as the vacuum tube amplifier 26 which in turn is energized by a potential drop across resistance 2| in circuit with the photo-electric cell IS] The high resistance 2| is in shunt to the grid and cathode of amplifier 26 over an obvious circuit. The cathode 2'! of this amplifier, as illustrated, is of the heater type, grounded at 24, and the current therethrough is kept constant by means of a l-ampere ballast tube 48. This cathode is heated by current from battery 28 and the latter may also furnish filament current for amplifier l! as illustrated. The plate circuit for amplifier 26 may be traced from anode 29, through meter 25 and its shunt 30, contact 3|, voltage regulator 32, terminal 33 connected to the positiveterminal of a 130-voltage source, the negative terminal of which is connected to the grounded terminal 34, through ground 24 to cathode 21. The voltage regulator 32 is useful but not essential and comprises ballast resistor 35 of 1,000 ohms in series with a gaseous discharge tube 36 across the power supply line.

In order to balance out the steady plate current which flows through meter 25, there is connected in shunt thereto, the battery 31 of 1 volts and the coarse and fine regulating resistances 38 and 39 respectively, although a single regulating resistance may be used instead.

In order to prevent inductive interference from associated apparatus, the amplifier 26 and meter 25 and their circuits as illustrated, may be inclosed in a metal shield illustrated by the dotted line 40 and grounded at 24.

The amplifier 26 and meter 25 and their power supply circuits may be housed in a cabinet 4| as illustrated in Figs. 2, 3 and 4. In Fig. 2, the conventional film recorder is illustrated with its lamp house 42, light valve 43 mounted on table 44 supported by legs 45. In this case, the cabinet 4| is mounted at the side of and below the standard film recorder, being fastened to and supported by the casing 46 in Fig. 3 which houses the amplifier Suitable straps 4'! are illustrated for this purpose.

As illustrated in Fig. 4 the cabinet 4| is provided with a door 41 on which may be mounted the amplifier 25 and its associated circuit elements. On the bottom of the door 41, as shown in Fig. 2, are mounted a control knob 49 to regulate the resistance 39, a control knob 50 to regulate the resistance 30 and a filament key 5|.

In order to better understand the difficulties overcome by the present invention as well as the various uses to which it may be put, it is advisable to consider in some detail the deficiencies in the prior methods and apparatus and the manner in which they have been overcome.

Until the development of the present type of clash and spacing indicator, no really quick and reliable means was available to the film recorder to check the spacing of his light valve during production. The method in use was to have the tuning and spacing of the valve checked before the start of the days work by a light-valve maintenance man, who used a microscope to space the valve ribbons to their normal separation of 1/1000 of an inch or 1 mil. At intervals throughout the days shooting the recorder would make a 1000-cycle, unbiased-clash level test of the valve, which, when compared with clashlevel tests made earlier in the day, served as an indication of whether the tuning, or more likely the spacing; of the valve had changed.

One unavoidable cause of change in tuning or spacing is that caused by a change in temperature of the valve ribbon and adjacent metal parts, which occurs in any valve over the first half hour of use, due to heat from the magnetizing winding and exciting lamp. This change in tuning is small, being in the order of 50 cycles, but there is considerable change in spacing, in some cases as much as 0.1 mil. or 10 per-cent of the initial spacing. To minimize this change in tuning and spacing due to the warming up of the valve, the light-valve maintenance man warms the valves before he checks their tuning and spacing, so that he adjusts them under conditions something like their operating conditions.

The other major cause of a change in valve tuning and spacing during production is caused by a slippage of the valve ribbons on their supporting bridges. caused by valve clashes or overloads from greater than normal ribbon excursion; that is. when the modulation exceeds 100 per-cent or the valve widens beyond 2 mils. Changes in valve spacing and tuning, caused by overloads, occur seldom during ordinary dialogue recording, but loud dialogue, shouts, gun shots and effects, and often the synchronizing clapper stick are of such a level as to cause considerable valve overload- At best, a recorder must wait untilthe end of the take to check his valve by means of the unbiased clash-level test, and this .3

may take time enough to delay the company two or three minutes before the recorder can be satisfied that the valve is in condition to proceed with the shooting. In the event that his valve has widened, he may decide to remove the valve and substitute another with some extra time being unavoidably necessary to set it up for recording. Even if he does find that the valve has, for example, widened, he still does not know where, in the take just completed, the valve widened, nor does he know just how much his noise reduction has decreased in that take, due to the widening, unless he takes time to make the socalled wheel check.

The wheel check resulting level of the tone imparted by the spinning of the wheel can be read on a volume indicator at the output of the photo-electric cell monitoring circuit. The level is read with the valve unbiased or normally spaced to 1 mil., and then read with the valve biased withdirect current. This reading is several decibels less, because the valve has narrowed correspondingly and less light is transmitted to the photo-electrio cell. The value of bias current is adjusted until the difference in the two level readings, as read by the volume indicator, is equal to the desired noise reduction in decibels. This wheel check takes a minute or two to perform and is an indication, mainly, of the value of bias current necessary for the corresponding noise reduction.

Some indication is given of a change in valve spacing if the amount of bias current required is found to have changed, but the amount can only be guessed at. Furthermore, the bias current change may have been due to a change in tuning, as well as spacing, or both. Some recorders have been able to check the valve spacing on wheel checks by noting it the level of tone transmitted by the valve in its unbiased position is constant from check to check. This is only reliable if the speed of the wheel is the same for. each reading and the gain of the photo-electric cell monitoring system remains constant. The speed of the wheel as now used varies with the degree of heating of the resistor in series with its electric motor, and the gain of the photo-electric cell monitoring system may change as much as one decibel, with changes in photo-electrio cell, photo-electric cell amplifier and monitoring amplifier battery supply voltages during a days operation, so that this method is not accurate enough to reliably indicate the spacing of the valve.

,The clash and spacing indicator 25 above described gives to the recorder at all times: (a) an accurate indication of the spacing of his light valve, (17') an accurate indication of the amount of noise reduction to be expected with the value of bias current being. used at the moment, and an indication of the noise reduction at any of the usual values of bias current, (0) an indication of the unbiased and biased clash levels, and an in- .dication of modulation overload readable as an average effective increase in valve spacing. Once the instrument is initially balanced, all of the above information is immediately available by reading the spacing, noise reduction, clash or overload on a single indicator meter, such as 25, by no more complicated operations than turning the bias-current key 52 on and off and adjusting the bias current, if necessary. The wheel check is no longer necessary. At any time between takes, or even in intervals of no modulation during a take, the recorder can tell at a glance the condition of his light valve.

. To be of any value whatever, a valve spacing indicator must be as accurate as commercial tolerances require, and more important, must maintain that accuracy over a days shooting at least. With this instrument the valve spacing can be read to 1 100,000 of an inch, and the corresponding change in negative density is just detectable with a densitometer. The best valves, at present, vary up to l/20,Q0O of an inch as an average change and up to l/5,000 of an inch in extreme cases with no, means heretofore available to measure that change while the valve was on the recording machine. The accuracy of the instrument is constant over a period of months.

The instrument is sensitive enough, so that with average valves of exciting lamp illumination, no light to full light through a valve spaced 1 mil. gives a full scale reading on the indicating meter 25.

The clash and spacing indicator, above described, consists of a single-stage direct-coupled amplifier 26, with a meter 25 in the output circuit, whose readings are directly proportional to the amount of light falling on the standard caesium type photo-electric cell l6 used in the monitoring circuit. The amount of light on the cell, in turn, for any given value of exciting lamp illumination is directly proportional to the spac-' ing of the ribbons of the light valve. Referring to Fig. 1, it can be seen that as the valve ribbons narrow in spacing, the photo-electric cell 16 receives less light, which causes less polarizing current to flow through the grid leak 21 in the grid circuit of the tube 26, which decrease in current is such that the voltage drop it causes across the leak is in a direction to make the grid less positive with respect to the cathode which, in turn, reduces the plate current flowing in the plate circuit of the tube 26. rent will be proportional to the change in cell illumination, providing the illumination-current characteristic of the photo-electric cell is a straight line, and providing the grid voltageplate current curve of the tube 26 is a straight I line over the range in which we are interested. The first condition is true with the change in light on the photoelectric cell encountered in recording and the second condition is true as use is made of a tube type 56 having a high mutual conductance for maximum sensitivity, with a straight grid voltage plate current characteristic as desired.

Tests were made on the type 56 tube 26 for various values of plate voltage, in order to find a convenient value to use which would give sufficient sensitivity and linearity with reasonable values of grid voltage and plate current. A value of 90 volts was decided upon as being the best suited.

Method of operation To set up the light valve 5, 6 for recording, using the clash and spacing indicator of this invention, instead of a clash bridge and} tone wheel, the procedure is as follows: The valve is first checked with a microscope and oscillator for a 1 mil. spacing of the ribbons and a tuning peak of 9,000 cycles. The valve is then placed on the recording machine and .the magnetizing winding, photo-electric cell amplifier switch, and exciting lamp turned on. The lamp current is set to give normal illumination. After a 15 to 30-minute warming up period, both the lamp current and the light-valve spacing should be settled and steady. This warming up period is necessary to bring the valve up to operating temperature, because of the slight change in spacing, due to the. heat from the magnetizing winding and exciting lamp. Furthermore, it allows the lamp and control rheostat to come to a steady temperature, so that the lamp current will be very nearly constant, and the initial peak voltage from the charged storage battery will be dropped to the more constant operating value. During. this time, the clash indicator has been warming up also. Now, the light from theexciting lamp is blocked ofi from the cell by holding a finger over the hole in the light valve plate. Thesensitivity control is turned clockwise to This change in plate curincrease the value of the resistance shunt 30 on the meter, and the fine balance control 39 is adjusted, so that the meter 25 reads zero. Upon allowing the light to pass through the valve and strike the photo-electric cell It once more, the plate current will immediately rise in the tube 26, as is indicated by the plate meter 25 rising to a high scale reading. For convenience the sensitivity control 35) is now adjusted such that the plate current reads 1 milliampere on the meter 25, which is a -1.5 milliampere meter. Now the value of current as indicated on themeter 25 corresponds to the amount of light striking the cell caused by normal lamp current and passed by a 1 mil.-spaced valve. Now the device is balanced for operation, the readings on the dial of meter 25 in milliamperes corresponding to theopening of valve 5, 6 in thousandths of an inch.

Determination of unbiased clash level Determination of bias current necessary for any desired noise reduction The tone is removed from the light valve by pulling out the plug from the 1000-cycle oscillator. With the input key (in series with the primary of transformer 9 and not shown) of the noise reduction control unit in the off position, the bias key 52 is turned on and the bias rheostat ll adjusted until the needle of the meter 25 moves down scale from 1 milliampere to the noise reduction reading desired. The meter is calibrated for 6, 8 and 10 decibels noise reduction, but any other values of noise reduction can-be used. The value of bias current, as indicated by the bias-current meter on the noisereduction control unit, is the current which will narrow the valve to a spacing which will give the amount of noise reduction indicated by the meter, i. e., by the clash and spacing indicator 25.

Theory of operation The theory of operation of the device is that for any value of illuminaton of the photo-electric cell l corresponding to a 1 mil. spacing of the valve 5, .6 and indicated on the meter 25, any reduction of illumination on the photo-electric cell It, caused by a narrowing of the valve ribbon 5, 5 will result in a proportionately decreased reading of the meter 25. For example, if the valve ribbons are, narrowed to 0.5 mil. or onehalf their initial spacing, the illumination of 7 the photo-electric cell will be cut in half, the

polarizing current flowing through the megohm resistance 2| in the grid circuitof the tube 26 will be but one-half its initial value, the voltage across it will be one-half its initial value, which will cause the initial change in plate current, indicated as l milliampere on the plate meter 25 to be reduced to one-half its value, or 0.5 milliampere. Such a reduction in light on the photo-electric cell [6 is equivalent to a voltage ratio of 2 to 1. or a noise reduction expressed in decibels of log 2 or 6 decibels. Therefore, the meter is marked 6 decibels noise-reduction at a current of 0.5 milliampere. Other values of noise reduction are marked accordingly.

Setting the gain of the noise reduction amplifier Use as overload indicator During normal recording, the light valve will vibrate around average spacings between the spacing corresponding to the set noise reduction a 1 mil. At any time when the bias current through the valve is zero, the meter 25 will indicate l milliampere. If at any time, when the valve is being modulated, the meter reads greater than 1 milliampere, the valve is clashing or overloading. The reason for this is that at levels of modulation greater than the clash level of the valve, the valve tries to move over a distance greater than from 0 to 2 mils, which corresponds to one hundred per-cent modulation. As the valve ribbons cannot close more than closed,

but can open more than 2 mils, an average amount of light strikes the photo-electric cell greater than that from a l-mil. valve spacing, and this greater amount of light causes the plate current to become greater than 1 milliampere. Accerdingly, the region on the meter above 1 milliampere is used to indicate overload, just asit was in the unbiased clash test, and that region is so marked as indicated in Fig. 5. The meter 25 is too sluggish to respond to the changes in plate current, due to the modulation .1

of the light valve by speech-frequency currents, but only indicates the average position of the light valve 5, 5 at any instant, according to the amount of bias current there is flowing at that moment through the ribbons. bias current can be turned off and modulation, due to speech or music, can be put on the valve, and the needle of the clash-indicator meter 25 will not move.

It will be understood, therefore. that the amplifier 25 is energized by the impulses to be recorded and also by an impulse depending upon the slow opening and closing of the light valve under control of the bias current. This ampliher 25 having a straight line characteristic, passes both sets of impulses to its output circuit in the same form in which it receives them, where the meter 25 in eifect eliminates the impulses to be recorded, as it is not responsive to alternating current and instead merely selectively responds to the impulse caused by the slow opening and closing of the light valve 5, 5.

Checking the spacing of the valve during recording The two positions of the valve which are of greatest interest are the unbiased and biased positions. After the valve is once set up, either of these positions may be seen at a glance by turning the bias oil and on. At the conclusion of To check this, the

a take, and after the mixer has closed his potentiometer to cut off any further modulation, the recorder normally records a short strip of biased and unbiased track by turning off his bias key. Just before and after he turns this key are good times to glance at the indicating meter 25 and note the positions of the valve ribbons- 5 and 6.. If the indications are. normal, he knows be yond question that the valve is ready for the next take. If the unbiased position of the valve is normal, but the biased position is not onythe proper noise reduction, the bias current. should be increased or decreased. until the spacing is correct. The indicating meter reads in tenths of a-mil. and fiftiethsof a mii., a haii of one of the smallest divisions being one one-hundredth of a mil. The zero balance of the instrument should be checked occasionally by blocking the light and seeing that the meter reads zero.

Exciting lamp check Just as any change in the valve spacing is shown by the indicator 25, so also, is any change in light striking the cell caused by a change in lamp current, This means that if the instrument is to be of value in checking the. valve spacing, a particular value of lamp current once set must be maintained. If a new value of lamp current is necessary, for any reason, it is, of course, only necessary to re-adjust the sensitivity control to make theunbiased reading read 1 milliampere with the new value of lamp current. If a lamp blackens during a days set-up, the clash indicator will note the decrease in illumination, for although a narrowed valve might indicate the same sub-normal reading, the valve could be removed and checked with the microscope. If the valve shows normal and the lamp current reads normal, then the lamp must be getting black, and the lamp current should be increased. In general, however, a lamp blackens so slowly that the density checks made in processing the film usually catch the under-exposure dueto a failing lamp. I

With a standard slit of 1 mil. width, valve spacings may be checked against the slit as a standard by substituting'the standard for the valve on any recording machine.

Using a calibrated standard exciting lamp and a standard l-mil. slit, a number of lamps may be accurately and rapidly calibrated in a few moments by merely substituting one for the other and adjusting the lamp current of each to get the same deflection on the indicating meter 25. Formerly, to calibrate lamps, an exposure had to be made for each lamp for each of a number of likely values of lamp current in the neighborhood of the suspected proper lamp current. These exposures had to be developed, and the density of the negative read with a densitometer, to see which value of lamp current gave the correct exposure to give the desired density.

The use of the clash and spacing indicator described has demonstrated that it has the following advantages:

l. Recordings made with the indicator show an almost constant density of the unbiased and biased track.

2. The noise reduction is held correct to less than 0.5 decibel.

3. Valve line-ups are simpler, more accurate and require less time.

4. At no time does the recorder need to take away the monitoring amplifier system from the mixer to check his valve, which results in delays and inconvenience. The condition of the valve is always known.

5. A widened or narrowed valve is noted immediately and can be replaced if it is thought necessary.

6. A broken valve isimmediately evident to the recorder by the much greater than normal reading of the indicator meter.

7. The amount of overload is evident to the recorder. even after his bias meter and the mixers volume indicator are no longer useful in indicating the amount of overload. If desirable, an extension meter exactly like the one on the clash indicator may be trunked to the mixer for his aid in observing level and overload values.

8. A defective lamp can be discovered by the recorder, which is now almost impossible with the existing indicating equipment, and which might result in a possible spoiling of a complete day's work.

9. Lamp calibrations can be made quickly and accurately by the recorder himself, if necessary, in the event that he might have a lamp fail and have no calibrated spares.

Having thus described the invention, what is claimed as new and desired to secure by Letters Patent, is:

1. In sound recording wherein a light valve is employed for modulating a beam of light in accordance with sound waves, the method of visually indicating the opening of said valve caused by biasing currents, which method comprises translating a portion of said beam of light into an electrical impulse, and translating said impulse into a direct visual indication of the extent of the opening of said valve in units of length caused by said biasing currents.

2. In sound recording wherein a light valve is employed for modulating a. beam of light in accordance with sound waves, the method of visually indicating the opening of said valve caused by biasing currents, which method comprises recording the modulated light beam, simultaneouslytherewith translating a portion of said beam of light into electrical impulses and translating said impulses caused by said biasing currents into movement proportional to the opening of said valve.

3. In sound recording wherein a light valve is employed for modulating a beam of light in accordance with sound waves, the method of visually indicating the opening of said valve, which method comprises recording the modulated light beam, simultaneously therewith translating :1 portion of said beam of light into electrical impulses corresponding to the impulses to be recorded and to impulses generated by the opening and closing of the valve under the influence of bias current, and translating said bias impulses into a visual indication of the extent of the opening of said valve.

4. In sound recording wherein a light valve is employed for modulating abeam of light in accordance with sound waves, the method of visually indicating the opening of said valve during sound recording operations caused by biasing currents, which method comprises recording the modulated light beam, simultaneously therewith translating a portion of said beam of light into electrical impulses caused by biasing currents, and translating said impulses into a visual indication of the extent of the opening of said valve.

5. In sound recording wherein a light valve is employed for modulating a beam of light in accordance with sound waves, the method of visually indicating the opening of said valve caused by biasing and overload currents during sound recording operations, which method comprises recording the modulated light beam, simultaneously therewith translating a portion of said beam of light into electrical impulses, and translating said impulses into a visualindication of the varying openings of the valve caused by said biasing currents and the supernormal opening of said valve when the valve is overloaded.

6. In sound recording wherein a light valve is employed to modulate a beam of light in accordance with sound waves, and wherein said light valve is biased in accordance with the audio currents being recorded, for noise reduction purposes, the method of indicating during recording the amount in decibels of the noise afiorded by said light valve, which method comprises recording the modulated light beam, simultaneously therewith translating a portion of said beam of light into electrical impulses caused by biasing currents and translating said impulses into a visual indication of the noise reduction in decibels.

7. The method of sound recording with a light valve having a bias thereon, which comprises translating the average valve opening due to said bias into a calibrated indication thereof while recording sound, and excluding from said indication the variations of the valve opening due to sound modulations thereof.

8. The combination of a light valve for modulating a beam of light in accordance with impulses to be recorded, a light sensitive device for monitoring modulations of said beam of light, means for recording during said monitoring, and means comprising a meter in circuit with said device for indicating the opening of said valve during the recording caused by biasing currents.

9. The combination of a light valve for modulating a beam of light in accordance with impulses to be recorded, means for biasing said valve, a light sensitive device for monitoring said beam of light during recording and means for indicating the average opening of said valve due to said bias during the recording.

10. The combination of a light valve for modulating a beam of light in accordance with imrecording, which method comprises translating a portion of said beam of light into an electrical impulse, simultaneously therewith recording said light beam, and translating said impulse into a visual indication of the opening of said valve during recording caused by biasing currents.

12. The combination of a light valve for modulating a beam of light in accordance with sound waves, means for varying the opening of said light valve in accordance with the envelope of the modulating sound waves, means for light impressing a light sensitive medium with said'light, and means operable during said light impressing for visually indicating the opening of said valve caused by the envelope of said sound waves.

. 13. The combination of a light valve for modu lating a beam of light in accordance with sound Waves, means for varying the opening of said light valve in accordance with the variation of the average value of said modulating sound waves, means for light impressing a light sensitive medium with said light, and means operable during said light impressing for visually indicating the opening of said valve caused by said variation in the average value of said modulating waves.

14. In a sound recording system, a light sensitive medium, means for varying the light on said medium in accordance with sound waves, means for controlling said varying means in accordance with the variation in the average value of said sound waves, and means operable contemporaneously with said first mentioned means for visually indicating said variation.

LLOYD T. GOLDSMITH. 

